- Home
- Conferences
- Conference Proceedings
- Conferences
5th International Congress of the Brazilian Geophysical Society
- Conference date: 28 Sep 1997 - 03 Oct 1997
- Location: São Paulo, Brazil
- Published: 07 November 1997
201 - 250 of 416 results
-
-
Slip Surfaces at Inner Corner Highs on the Mid-Atlantic Ridge
Authors B. Janssen, D. Blackman and Joe CannWe present recently collected high-resolution data from the Atlantis Fracture Zone which is situated on the Mid-Atlantic ridge at 30° north. The Sirnrad multibeam bathymetry has ground resolution of 50-100m and the deep-towed side-looking sonar device (TOBI) has a resolution of 3-6m. The combination of imagery proved a powerful tool for interpreting the Ridge Transform Intersections (RTI).
-
-
-
Some values of Effective Elastic Thickness determined for the South American Plate
Authors M.S.M. Mantovani, S.R.C. de Freitas and W. ShukowskyIn the last decades, the effective elastic thickness (Te) has been considered an important parameter for the study of the lithosphere rheology.
-
-
-
Flexural Rigidity at Parnaíba Basin - Brazil
Authors R.M. Vidotti, C.J. Ebinger and J.D. FairheadThe intracratonic Parnaibá basin, located in Northeast Brazil, covers an area of approximately 600,000 km2. It is surrounded by Brazilian cycle (700-500 Ma) orogenic belts and its sedimentary cover ranges in age from Silurian to Cretaceous. These sequences have been intruded by volcanic rocks of Early Jurassic and Early Cretaceous ages. The maximum sedimentary thickness is around 3500 metres, based on surface geology, geochemical surveys, exploratory wells and seismic sections (Góes et al, 1993).
-
-
-
Subsidence Modeling of the Parnaibá Basin (NE Brazil)
More LessThe tectonic subsidence of the Parnaiba Basin was estimated through systematic backstripping using 22 boreholes which reached the metamorphic or sedimentary basement. A non-uniform lithospheric tretching model was used as a first approximation for the modeling of the tectonic regime. This procedure was suggested by the presence of several grabenlike structures, as confirmed by a few shallow seismic sections, exploratory boreholes and gravity maps.
-
-
-
Regional Gravity and Aeromagnetic Investigation of the Takutu Basin, Roraima - Brazil
Authors J.V. Miranda, C.J. Ebinger and J.D. FairheadA regional gravity study has been undertaken over the Takutu Basin, in Roraima-Brazil, sponsored by the ABGP (Anglo Brazilian Gravity Project) with logistical assistance provided by the University of Leeds, Earth Sciences Department, the ON (Observatório Nacional do Rio de Janeiro-Brazil) and the CPRM (Companhia de Pesquisa de Recursos Minerais-Brazil). The gravity survey carried out in this region helps to delineate tectonic provinces and to define the boundaries of the Mesozoic Takutu rift basin. The only existing gravity data available, prior to this work, was part of a country-wide data acquisition project from the city of Manaus along the road to the Venezuelan border, forming a long N-S profile. A new set of 291 gravity measurements, with 3km spacing was established using a Lacoste & Romberg gravity meter.
-
-
-
Caracterizaçăo Geológica e Geofísica de uma Estrutura na Borda Leste do Graben de Umbuzeiro, Bacia Potiguar/RN, e Implicaçőes Neotectônicas
More LessRecentemente, vários estudos de cunho neotectônico estăo sendo realizados no Estado do Rio Grande do Norte, com o objetivo de melhor compreender a deformaçăo atual da crosta, caracterizando as estruturas produzidas e os regimes tectônicos envolvidos. As ferramentas utilizadas nesses estudos săo variadas, destacando-se as análises de mecanismos focais, de rede hidrográfica, de geomorfologia, de fotosensores e levantamentos geológico-estruturais.
-
-
-
Surface-wave Tomographic Inversion of the South American Lithosphere (The SISAL Project)
Authors J.A. Rial, O.Y. Vdovin, M.H. Ritzwoller and A.L. LevshinWe are studying the dispersion characteristics (measurements of group velocity) of broad band (15-2oosec) Rayleigh and Love waves propagating across the South American continent and adjacent oceans. The ultimate purpose is to invert for a high resolution, tectonic scale (lateral:5ookm) crustal and lithospheric model covering the South American, Cocos, Caribbean, Nazca and Scotia plates to depths of approximately 200 km. To date, data from more than 400 events have been analyzed yielding in excess of 4100 paths across the continent and neighboring regions. These measured group velocities are summarized in the form of tomographic maps for group velocities of Love and Rayleigh waves with periods ranging from 20 to 150 sec. We report the results of a resolution analysis (checker-board test) for a variety of periods for both Rayleigh and Love waves. The estimated group velocity maps are compared with those predicted from a recent model in which the crust is CRUST-5.1 (Mooney et. al., 1996) and the mantle is S16B30 of Masters et.al., (1996).
-
-
-
Topographic and Structural Expressions of the Ongoing Compression in Midplate South America
New borehole breakouts and focal mechanisms confirm previous results obtained for the World Stress Map and greatly improve our knowledge of the midplate stress field: present-day horizontal compression prevails throughout midplate South America (Assumpcao, 1993; Lima et al, in press). Preliminary geodetic space-based results obtained from the French system DORIS have shown that, for the 3- year interval of observations, the ten available base lines are all shortening. The absolute velocities of the three stations located on the western edge of the plate are oriented to the east, suggesting that most of the shortening is absorbed by the Andes. Nevertheless, two other base lines that do not cross the Andean edge (between Kourou, French Guyana, and Cachoeiro, Southeastern Brazil; between Rio Grande, Southernmost Chile and Kourou) indicate that shortening is also occurring in a midplate setting.
-
-
-
First Determinations of the Crustal and Upper Mantle Structure in the Amazon from Seismological Data
Authors Joăo Willy Corręa Rosa and Frank ScherbaumWe have installed a temporary seismological array of three broadband stations in a north-south trending line from the Pitinga (PTGA) IRIS station to the city of Manaus. This is a summary of the first stage of a long-term project for the study of the geologic structure of the Amazon Basin, between the Volkswagen Stiftung, the Universitiit Munchen, GFZ-Potsdam and the University of Brasilia. Since the Pitinga (PTGA) IRIS station is located on the shield right at the border of the Amazon Sedimentary Basin, and Manaus is located on the center of the basin, the purpose of this study is to show a first picture of the variations observed in the crustal and upper mantle structure of the Amazon Basin.
-
-
-
Crustal Thicknesses in SE Brazilian Shield with Receiver Function: Isostatic Compensation by Density Variations in the Lithospheric Mantle.
Authors Marcelo Assumpçăo, David James and Arthur SnokeThe Brazilian Lithosphere Seismic Project (BLSP: University of Sao Paulo / Camegie joint project, 1992-1996) operated temporary broad-band stations in 19 different sites in the southeastern Brazilian shield. Crustal thicknesses were estimated for 16 sites using receiver functions. For each station, receiver functions were stacked for different sets of earthquakes according to azimuth and distance. The Ps, Moho converted phase, was clearly identified in most stations. Crustal thicknesses were estimated using an average crustal P-wave velocity of 6.45 km/s. VpNs ratios were used as 1.70 for the stations in the Sao Francisco craton and adjacent fold belt (determined from small, local earthquakes), and 1.73 for the other stations. Crustal thicknesses ranged from 37 to 47 km. Although there is a clear normal correlation between topography and Bouguer anomaly in the study area, Moho depths show the opposite pattern from the expected: areas of low topography and less negative Bouguer anomaly, such as the Parana basin, have thicker crust (40 to 47 km) compared with the high elevation areas of the craton and fold belt (38 to 43 km). This may indicate that lower density (~p about -0.04 g/cm3) of the lithospheric mantle under the Archean block of the Sao Francisco craton is required for the regional isostatic compensation. In fact, surface wave dispersion seems to require slightly lower velocities in the upper mantle beneath the craton as compared with the Parana basin. Alternatively, if the density contrasts between Archean and Proterozoic lithospheres is smaller than the values above, then the crust beneath the Parana basin may be denser compared with the craton and fold belt. This would be consistent with magmatic underplating in the lower crust beneath the Parana basin. However, reasonable densities anomalies « 0.06 g/cm3) in the Parana crust would still require low density for the Archean lithosphere
-
-
-
Some Questions Regarding Methods to Image the Crust at the Ocean Continent Boundary
By M. TalwaniThe need to integrate geophysical data - gravity, magnetic, and seismic - to image the crust at the ocean continent boundary is obvious. In this paper we shall briefly discuss the seismic method
-
-
-
Seismic Bright Spots, Magmatism, Rheology and Moho Tectonics: New Results from Crustal Seismic Profiling
Authors Larry D. Brown, Andrew Ross and Camelia DiaconescuRecent seismic reflection profiling in Tibet (Project INDEPTH) has discovered some the latest examples of anomalously strong seismic reflections, or "bright spots" in the deep crust. These reflections, characterized by negative reflection polarities, strong shear wave reflectivity, low bulk shear wave velocities and high local conductivities, are interpreted to be fluid, probably magma, trapped in the basement.
-
-
-
Laminated Lower Crust and Other Reflectivity Patterns
More LessSeismic reflection profiles provide the highest resolution on the in situ structure of the lower continental crust. Reflectivity patterns correlate with distinct geologic settings. There is a tendency of the rigid upper crust to store the memory of old collisional belts, showing ancient thrust-and shear zones.
-
-
-
Some results of Investigating the Crustal and Upper Mantle Structure in Central Europe by Means of Seismic Surface Waves
More LessThe Earth's crust in central Europe has been investigated in detail along several intemational profiles of deep seismic soundings. Investigations of surface waves allowed to extend the structural investigations into the upper mantle. Pronounced lateral inhomogeneities in the upper mantle were confirmed by the surfacewave method. Moreover, short-period surface waves, genetated by explosions, were used in studying the superficial structure of several sedimentary basins.
-
-
-
The Observation and Explanation of Asymmetrically Reflected PP and SS Precursors
Authors Martin Schimmel and Hanneke PaulssenSince more than 30 years symmetrically and asymmetrically reflected phases are observed in Pwave coda of seismic recordings for distances ranging between 80 deg and 120 deg. These phases appear before the PP-phase and are therefore referred to as PP precursors. The symmetrically reflected precursors are known as PdP phases and have their reflection point at depth d midway between source and receiver.
-
-
-
Microtemperature Signals of the Earth's Crust
More LessTemperature variations in the uppermost crustal layers result from several external as well as internal effects. These variations superpose the basic and rather stable energy flow from the earth's interior. Variations of the surface temperature, caused by diurnal, annual or long-term waves, jerks or other changes, dissipate into the subground to different extents and may be detected by measurements in the subground. Also environmental conditions, e.g. the cutting of tropical forest, cause a variation of the subsurface temperature as demonstrated with measurements.
-
-
-
Petrology and Geochemistry of the South Atlantic Ridge from 0° to 42° S
More LessPetrological and geochemical variations along the South Atlantic Ridge show that eruption temperatures for the South Atlantic Ridge primitive basaltic glasses (MG#>64) decrease systematically along the ridge to the south from 0° to 42°S which appears to be related to the lower mantle temperature gradient. The lower mantle (below the 650 km discontinuity) beneath the South Atlantic shows a lower seismic P wave velocity with a gradual increase southward (around 42°S) (Dziewonski, 1984). This seismic velocity is related to temperature, or degree of melt, where higher seismic P wave velocities indicate colder material. The lower mantle temperature also seems to play an important role in controlling the degree and volume of melting along the South Atlantic Ridge.
-
-
-
Large Igneous Province
More LessLarge igneous provinces (LIPs), voluminous emplacements of predominantly mafic extrusive and intrusive rock, originate via processes other than those responsible for the creation of crust in the plate tectonic cycle, i.e., 'normal' seafloor spreading and subduction. They are produced when ascending hot plumes of mantle material reach the upper crust, and are generally considered to be relatively or totally independent of the movement of the Earth's lithospheric plates. LIPs and plumes therefore manifest a mode of mantle circulation not accounted for in the existing plate tectonics paradigm.
-
-
-
Effects of Small-scale Convection in Igneous Crust Production at Rifted Continental Margins
Authors R.R. Boutilier and C.E. KeenObservations of large igneous crustal thickness at rifted volcanic margins has led to suggestions that small scale convection in the upper mantle may be in part responsible for increasing the delivery of magma to the overlying lithoSphere. We have quantitatively assessed this concept by numerical modelling H271of the viscous flow caused by divergent plate motions. The model consists of a viscous, temperature and pressure dependent, non-linear fluid whose behaviour is found by solving the Navier Stokes equations for incompressible flow in two dimensions using a finite element formulation.
-
-
-
Were There Moving 'Plumelets' in the South Brazilian Continental Lithosphere
By V.M. HamzaIt is proposed that melt fronts generated by Tristan da Cunha plume during Cretaceous times is the deep thermal process responsible for the observed features in the spatial distribution of heat flow anomalies and alkaline magmatism in southern Brazil. Model studies indicate that plume derived melts can move over large distances without losing much of their thermal energy. Changes in the flow rates of such plumelets along networks of deep lithospheric channels can account for the observed age pattern of alkaline intrusives. It is likely that melt fronts are currently located at or near areas of high heat flow and suitable geoelectrical sounding methods should be capable of detecting their presence.
-
-
-
Tectonic Evolution of the Rift Basins in the Northeastern Brazilian Region
Authors W.U. Mohriak, M. Bassetto and I.S. VieiraThis work discusses the tectonic evolution of the Tucano and Sergipe rift basins onshore and offshore the northeastern Brazilian region, based on integration of deep seismic reflection profiling, potential methods, geological data, and tectonic analysis. We address the following themes: a) the geologic evolution of the rifts; b) the potential methods response to the deep structure of the rifts; c) the seismic expression of major tectonic features; and e) a possible geodynamic model for the evolution of the rift system.
-
-
-
Marine Geology and Geophysics of the Eastern Mediterranean
Authors J. Mascle and E. ChaumillonThe Eastern Mediterranean sea appears to be a relict oceanic space between two major continental plates Europe to the north and Africa to the south. Today this marine space includes two relatively deep basins: the oceanic crust floored Ionian sea to the west, believed to be of Mesozoic age and the Levantine basin where some remnants of a triassic oceanic crust have been indicated. The major morphostructure of the area are however the so-called Mediterranean Ridge, the Nile deep sea fan and the Cyprus outer arc.
-
-
-
SAGE (Summer of Applied Geophysical Experience) Sets New Trends in Geophysics Education
More LessSAGE (Summer of Applied Geophysical Experience) focuses modern geophysical methods on major geologic problems using seismic reflection and refraction, gravity, magnetics, electrical resistivity, magnetotellurics (MT), and other electromagnetics. The four week field format includes approximately equal portions of hands-on field work, lectures/discussion, and computerprocessing/interpretation. The field area is in and adjacent to the Rio Grande rift in the State of New Mexico, USA. Major explosion seismic and vibroseis efforts, gravity, and MT have been successfully directed at understanding rift processes. Recent studies have applied shallow geophysics to local groundwater investigations and environmental problems at the Los Alamos National Laboratory (LANL).
-
-
-
Implicaçős Tectônicas da Curva de Deriva Polar Aparente Mesozóica para a América do Sui
More LessEstudos paleomagnéticos provaram ser uma ferramenta poderosa na quantificaçăo dos deslocamentos das placas litosféricas, bem como em problemas de deformaçőes regionais. Rotaçőes crustais em relaçăo a qualquer eixo podem ser detectadas pelo paleomagnetismo, destacando-se as rotaçőes in situ, ou segundo eixos verticais, que de outra forma năo seriam facilmente identificáveis. Entretanto, para que isso seja possfvel, é necessário que os dados de regiőes deformadas sejam comparados a dados de áreas cratônicas. Estes dados levam ao cálculo de pol os paleomagnéticos de referęncia. A sequęncia de polos de referęncia de diferentes idades, vern a ser a curva de deriva polar aparente (CDPA) para urn bloco continental e deve refletir somente os deslocamentos da placa litosférica através do tempo.
-
-
-
The Neoproterozoic Rodinia, its Break-up and the Formation of Gondwana: Paleomagnetic Evidence.
Authors M.S. D‘Agrella-Filho, R. Siqueira, R. Trindade, C.F. Ponte-Neto and I.G. PaccaHoffman (1991) elaborated a model proposing the Meso/Neo Proterozoic Rodinia supercontinent (Fig. 1), based on geological and (partly) on paleomagnetic grounds. He suggested that Baltica, Amazonia, Congo and Kalahari were the most probable units to border eastern and southern Laurentia, since they have similar Grenvillian orogenic belts (1.2-1.0 Ga). In his model, Hoffman (1991) suggests that the (present) northeastern Kalahari craton (including the Lurian belt) was flanked by East Antarctica, and that this craton belonged to the East Gondwana during the amalgamation of the Gondwana supercontinent in the Neoproterozoic.
-
-
-
Nova Metodologia para a Determinaçăo da Suscetibilidade Magnética e da Magnetizaçăo Remanenre de Rochas
More LessUsualmente a determinaçăo da suscetibilidade magnética de rochas e efetuada em ponte de susceptibilidade utilizando-se corpos de prova cilíndricos de 1"x 2" e para a medida deste parâmetro físico em direçăo ortogonal é necessário a confecçăo de outro corpo de prova. A precisăo deste tipo de determinaçăo é, na prática, usualmente de 5 a 10% com sensibilidade de l x l O" u.c.g.s. No caso de nao de dispor de amostras de tamanho suficiente para a elaboraçăo de tais corpos de prova cilíndricos, pode se utilizar de fragmentos de rocha, mas neste caso a precisăo cae. Por este motivo, e para poder medir as propriedades magnéticas em pequenas amostras, o autor propôs uma nova metodologia para determinaçăo deste parâmetro em gemas, e rninerais (Haralyi 1993, 1994) cuja abrangencia e agora estendida para rochas é ensejando determinaçăo com maior precisăo e repetibilidade.
-
-
-
Estudio Paleomagnetico de La Formacion Tipilihuque (Alumine, Argentina): Un Unico Evento Volcanico?
Authors G.H. Ré, O. Ruffo and R. TomezzoliEn la región comprendida entre Zapala y Junin de los Andes (prov. de Neuquén), se observan importantes afloramientos de secuencias basálticas de edad cenozoica; estas secuencias fueron ordenadas estratigráficamente como Basaltos del cero al siete; los cuales abarcan desde el Oligoceno (Basalto 0) al Holoceno (Basalto VII); y se los puede agrupar en dos grandes períodos magmáticos: uno pre- Mioceno y otro al Plio-Holoceno.
-
-
-
Correlaçăo Geotectônica de Dados Aeromagnéticos e Gravimétricos no Norte do Cráton do Săo Francisco Regiăo Nordeste
More LessNa literatura especializada já é fato consagrado a utilizaçăo de dados aeromagnéticos e gravimétricos para interpretaçőes tectônicas. Os dados geofísicos fornecem um visăo em profundidade que corresponde a uma forma simplificada da terceira dimensăo dos dados geológicos, tornando-se imprescindível em áreas de escudos e faixas dobradas pré-cambrianas pouco conhecidas ou parcialmente encobertas por sedimentos fanerozóicos. O objetivo desta comunicaçăo é definir o arcabouço geotectônico de uma área complexa, caracterizada pela contigüidade de tręs províncias geológicas importantes, o Cráton do Săo Francisco (CSF), a Província Borborema (PB) e a Bacia do Parnaíba (BP), (Fig. 1).
-
-
-
Estruturaçăo das Associaçőes Petrotectônicas do Escudo Sul-Rio-Grandense: Caracterizaçăo Geofísica Através da Magnetometria, Gravimetria e Gamaespectrometria
Authors A.F. Uberti Costa and L.A. D‘Ávila FernandesImportantes definiçőes referentes ŕ compartimentaçăo das associaçőes petrotectônicas encontradas no Escudo Sul-Riograndense foram obtidas através da análise de dados magnetométricos, gravimétricos e radiométricos. Essas informaçőes, que permitem também a observaçăo em profundidade de feiçőes geológico-estruturais significativas, e que pela primeira vez săo examinadas em conjunto, cobrindo toda a extensăo do Escudo, vern contribuir consideravelmente para urn melhor entendimento dos processos de estruturaçăo da crosta continental dessa regiăo.
-
-
-
Estrutura Crustal do SuI e Sudeste do Brasil Inferida a Partir de Dados Aeromagnéticos e Geotérmicos
Authors P.L.F. Rocha, L.F.S. Braga and V.M. HamzaA utilizaçăo de dados aeromagnetométricos é de fundamental importância na exploraçăo de recursos minerais e na exploraçăo de petróleo, sendo empregado para definiçăo de lineamentos e faixas crustais em escala continental até a caracterizaçăo de detalhes estruturais dentro dos sedimentos.
-
-
-
Correlaçăo entre Condutividade e Dilataçăo Térmica em Rochas Metamórficas da Regiăo de Săo Sebastiăo, Estado de Săo Paulo
Authors R.P. Pacheco and V.M. HarnzaAs propriedades térmicas dos solidos, tais como a condutividade e dilataçăo, săo geralmente dependentes de diversos fatores intrínsecos e ambientais. No caso de materiais geológicos, os fatores irnportantes săo: composiçăo mineralógica, estrutura cristalina, porosidade, pressăo e temperatura. Conhecimentos das influęncias destes fatores e das inter-relaçőes entre as propriedades termofísicas săo considerados básicos nos estudos do regime térmico do interior da Terra.
-
-
-
Infravermelho Termal ( 8 - 12 urn ) na Caracterizacao de Rochas Vulcânicas
Authors A.P. Pachęco and Í. VitorelloEmissividades e inércias térmicas de amostras de rochas vulcânicas da Bacia do Paraná foram determinadas em Iaboratório por métodos radiométricos no infravermelho termal (8 -12 µm). Relaçőes lineares da emissividade e inércia térmica com Si02 mostraram nítida diferenciaçăo entre os litotipos básico e ácido e tendęncia de separaçăo no litotipo básico de amostras do norte e suI da Bacia.
-
-
-
Domínios Estruturais da Porçăo Oceânica da Margem Sul-Sudeste Brasileira
Authors M. Bassetto and F.F. AlkmimO estudo dos dados sísmicos e potenciais do Projeto LEPLAC (Plano de Levantamento da Plataforma Continental Brasileira), no âmbito da porçăo oceânica da Margem Continental Sul-Sudeste do Brasil, revelou significativa compartimentaçăo morfoestrutural da área (Souza et al. 1992; Bassetto 1997). Natureza das estruturas dominantes do embasamento, espessura crustal ígnea, feiçőes do embasamento acústico e relevo do fundo oceânico săo parâmetres que exibem variaçőes consideráveis e relativamente ordenadas ao longo do setor estudado.
-
-
-
Caracterizaçăo Geotérmica no Campus da Universidade do Amazonas
Authors P. Noronha Serra, R.L.C. Araujo, J.S. Carvalho and W.S. Marques PalhaDurante os meses de janeiro a dezembro de 1996, com o objetivo de verificar as variaçőes diurnas e sazonais de temperatura, foram realizadas, diariamente, medidas de temperatura em dois furos, denorninados respectivamente de Furo 1 e Furo 2, nas profundidades de 0,0m, 0,5m e 1,0m, sendo um sem (F1) e o outro com cobertura vegetal (F2). Tais medidas foram efetuadas nos horários das 10h e 18h. As medidas a 0,0m foram realizadas com o auxílio de um termômetro, cujo sensor é um termopar, com precisăo da ordem de ± 0,05°C. Nas de 0,5 m e 1,0m foram utilizados termômetros de termistor, com precisăo da ordem de ± 0,005°C. O presente trabalho é de fundamental importância para dar continuidade aos estudos sobre a influęncia da incidęncia da radiaçăo solar na superfície terrestre e da posterior perturbaçăo térmica propagada para as camadas subsuperficiais (Araujo, 1987; Carvalho, 1981; Souza et al., 1990;), bem como as variaçőes de temperatura decorrentes desse processo.
-
-
-
Velocidade de Transmissăo da Perturbaçăo Térmica Superficial
Authors R.L.C. Araujo, J.S. Carvalho, W.S.M. Palha and P.N. SerraNa atualidade é intensamente difundido que a atividade de desmatamento na Amazônia provoca diversos distúrbios ambientais, a níveis local, regional e global (Souza et aI., 1996; Souza et aI., 1989; Araujo et aI., 1995). No entanto, năo há, ainda, informaçőes quantitativas suficientes para se elaborar urn modelo, plenamente aceito, o qual descreva, detalhadamente, a influęncia da cobertura vegetal sobre o regime geotermal raso, ou seja, sobre a estrutura geotérmica nas camadas mais superficiais da Terra. Tal conhecimento é de suma importância, face ao fato de que flutuaçőes geotermais em tais camadas acarretam consequęncias diretas na biosfera terrestre, como também, ser tal zona de fundamental interesse na prospecçăo mineral. Os resultados constantes neste trabalho săo relevantes no contexto da geoterm a, pois permitem o melhor conhecimento sobre a estrutura geotérmica rasa.
-
-
-
Estudo Acerca das Influęncias Sobre os Valores de Condutividade Térmica
More LessNa regiăo Amazônica, a estrutura geotermal rasa sofre mensuráveis perturbaçőes térmicas, devido principalmente a grande incidęncia da radiaçăo solar na superfície terrestre. Além deste, outros fatores podem regular o comportamento térmico de uma área, tais atores săo: condiçőes do solo, precipitaçăo pluviométrica, evapotranspiraçăo, etc. Devido aos efeitos deste fatores, săo poucos os trabalhos de geotermia que consideram a zona situada desde zero até cerca de quarenta metros de profundidade, sendo a grande maioria dos estudos realizados a maiores de profundidade, a partir de onde considera-se que os efeitos transitórios năo săo mensuraveis.
-
-
-
Variaçőes do Fluxo Geotermal Raso em Áreas Com e Sem Cobertura Vegetal: Um Estudo de Caso na Regiăo Amazônica (Brasil)
No presente trabalho foi realizado urn estudo sobre o regime geotermal raso na área do Campus da Universidade do Amazonas, no município de Manaus, no período de setembro/95 a junho/96. Os dados necessários para a realizaçăo deste trabalho foram coletados em dois locais, sendo um com e outro sem cobertura vegetal, a fim de melhor verificar as influęncias da radiaçăo solar incidente nos mesmos. Consideramos este trabalho de suma importância no âmbito da geotermia rasa na Amazonia, pois seus resultados permitiram urn maior entendimento da estrutura geotermal rasa; a determinacao da faixa de variacao do fluxo termico raso, alem de tomar possivel efetuar previsoes geotermicas.
-
-
-
Relationship between Regional and Local Heat Flow in a High Enthalpy Geothermal Field-los Azufres, Mich, Mexico
More LessIn the last five years growing interest in low enthalpy and medium size geothermal prospects in Mexico increased the institutional support for thermal methods research. The Comision Federal de Electricidad (CFE) decided to analyze temperature data taken during drilling stops in boreholes of well known geothermal zones, to analyze the order of magnitude and characteristics of thermal conditions at different locations, relative to the faults acting as heat and mass flow channels, with the goal to define interpretative criteria for the study of less known areas.
-
-
-
AMAZONScope: A BB Digital Seismograph Network for Monitoring Seismicity of the Brazilian Amazon
More LessTwenty years ago very little was known about the seismic activity of the Amazon region. The low population density and the delay in the installation of seismograph stations in the region contributed to the lack of knowledge about possible earthquakes. This situation only changed in the late 70's when studies about the seismicity of the Brazilian Amazon region were initiat+ed+H308 with the progressive installation of a network of stations through co-operative agreement between the University of Brasilia and Eletronorte (Electric Plants of Northern Brazil), The number of earthquakes that have already been recorded and the known cases of reservoir induced seismicity (RIS) show that the region bears an important level of seismic activity which needs to be carefully evaluated. To improve the capability of-seismic detection in this region, which represents 60% of the national territory, a modem network of stations, named AMAZONS cope, was conceived. Its purpose is to perform a continuous seismic monitoring program. The Network installation, operation and data analysis and interpretation, will be done by the University of Brasilia (UnB) Seismological Observatory (SIS) in collaboration with Eletronorte and of the Amazonian Military Command (CMA).
-
-
-
Recent Seismic Activity in the S-W Edge of the Mato Grosso-Goiás- Tocantins (MGT) Belt (Central Brazil)
More LessIn Central Brazil lies one physiographic province with outstanding geological, geophysical and seismotectonical features, it is known as Goias- Tocantins seismic belt (Assumpcao et al. 1986; Veloso 1988; Fernandez et al. 1991), but for reasons to be explained bellow, we advance the more appropriate denomination Mato Grosso-Golas- Tocantins seismic belt. This physiographic province (Fig. 1) is a Proterozoic orogen formed by an inversion of an oceanic basin, provided by the convergence between the Amazon and the San Francisco-Congo cratons during formation of the eastern part of the Gondwana super-continent (Fuck et al. 1993, 1994; Fuck 1994). The present dynamics of this province is represented, among other factors, by the conspicuous seismicity ranging the microearthquake to small earthquake levels (Fig. lA) , that present a NE to SW trend, extending from Tocantins state (in north) to Mato Grosso state (in south)[hence, we consider more suitable to call it Mato Grosso-Goias-Tocantins seismic belt (MGT belt)]. During the beginning of 1996 a salient seismic sequence originated some 10 to 15 km north of the small town of Araguaiana (Mato Grosso). This paper is intended to introduce a brief presentation of the main features and findings related to this earthquake sequence.
-
-
-
Reservoir Induced Seismicity at Nova Ponte (MG): Revisited
Nova Ponte reservoir (situated on the Araguari and Quebra Anzois rivers, planned capacity 1.28 x 109 m3 , maximum water depth 132 m; formed behind an earth-rockfill type dam with height 142 m and length 1600 m) is one of the recent cases of reservoir induced seismicity (RIS) in Brazil (see also Veloso et al., 1994 and 1995 or Ferreira et al., 1995 for presentation of other case histories of RIS in Brazil). Pre-impounding seismic monitoring has started 8 years before filling, using a single l-component SP analog seismic station, without detecting any local seismic activity in a radius of 80 km around the dam. Shortly after beginning of impounding (14 October 1994) quasi-continuous induced activity was detected. The seismic activity was triggered as soon as the lake attained about 20% of the full planned volume.
-
-
-
Correlaçăo entre a Sismicidade e a Estrutura da Bacia Potiguar, NE do Brasil
Authors J.M. Ferreira, J.A.M. Moreira, R.T. Oliveira, M.K. Takeya and M. AssumpçăoA borda da bacia potiguar é de há muito reconhecida como uma das mais importantes regiőes sísmicas do país (Berrocal et al., 1984). No entanto, até o momento, năo se tentou ainda correlacionar a atividade sísmica com a estrutura da bacia, 0 que será feito neste trabalho.
-
-
-
As Assinaturas Sísmicas Crustais da Sub-Bacia do Jandiatuba (Bacia do Solimőes)
Authors R.J. Vega Sacasa and J.B. Sena CostaO presente trabalho é uma síntese das informaçőes geológicas e geofísicas da parte ocidental da Bacia do Solimőes, denominada de sub-bacia do Jandiatuba. A análise de informaçőes da subsuperfície săo representados por dados de sísmica profunda na forma de uma seçăo sísmica com registro de 20 segundos, localizada na parte central da sub-bacia do jandiatuba, assim como da compilaçăo da literatura intemacional relacionada com programas de levantamentos a grandes profundidades realizadas em diferentes partes do mundo como o COCORP, BIRPS, DEKORP e ECORS. Estes resultados foram relevantes para fins de comparaçăo e correlaçăo com o objetivo de contribuir no entendimento geotectônico da refletividade desse segmento crustal da amazonia.
-
-
-
Determinaçăo da Espessura Crustal na Margem Ativa da Regiăo Antártica
Authors L.C. Torres, B.S. Gomes and L.A.P. GamboaO propósito de presente trabalho consiste na apresentaçăo de urn modelo capaz de contribuir para determinar a espessura crustal da regiăo compreendida pela fossa Shetland do Sul, arquipélago vulcânico Shetland do SuI e bacia Bransfield, na regiăo Antártica. Para alcançar tal propósito, foi utilizado urn perfil geofísico contínuo (sísmica multicanal, magnetometria e gravimetria) com 393km de extensăo, coletado pelo Brasil durante os anos de 1987 e 1988. O perfil gravimétrico observado, após reduçăo de latitude e Eötvös, gerou a gravidade ar livre, a qual foi comparada com o perfil gravimétrico produzido pelo modelo proposto. Através da análise dos resultados, pôde-se comprovar a complexidade da geologia regional, apresentando uma tectônica compressiva no limite convergente de placas e uma tectônica distensiva no interior da bacia Bransfield.
-
-
-
Sistema de Correçăo de Tempo, Baseado em GPS, para Estaçőes Sismográficas Analógicas e Digitais
O Observatório Sismológico (SIS) da Universidade de Brasília (UnB), em parceria com a iniciativa privada, desenvolveu e implementou um projeto de sistema de aquisiçăo de tempo preciso para estaçőes sismográficas analógicas e digitais. O sistema funciona utilizando informaçőes de tempo e pulsos de IPPS ( um pulso por segundo), provenientes de um GPS (Global Positoning System). Fornece como aídas informaçőes de hora, minuto, segundo e sinais de tempo codificados. A preciçăo e de um milisegundo. A execuçăo deste projeto resolveu, de forma relativamente barata, um antigo problema relacionado com a falta de urn sistema de tempo, que fosse, barato, preciso e apropriado para estaçőes sismográficas analógicas (Barros et al, 1991) e arranjos sismográficos digitais (Barros et al, 1993 e Barros 1995).
-
-
-
Inversăo 2-D para a Distribuiçăo Global da Velocidade de Grupo das Ondas Rayleigh
Authors A.N. de Castro Santa Rosa and J.W. Corręa RosaDesde 1960, tem-se observado grande desenvolvimento técnico dos instrumentos com PRESS (1956) e SANTO & SATO (1966). Os registros disponíveis das estaçőes digitais săo obtidos das redes como: GDSN, IDA, IRIS e GEOSCOPE. Com todo este avanço, a precisăo das medidas cresceu e, como conseqüęncia disto, a tomografia utilizando dados de período longo, pode ser usada para, em grandes scalas, observar a heterogeneidade lateral de modelos na Terra. Temos, como exemplos destes trabalhos: SOURIAU & SOURIAU (1983), onde foi considerada a inversăo de dados de velocidade de fase semelhante foi o trabalho de ROSA (1986). Neste trabalho observamos as anomalias de velocidade de grupo relacionadas as grandes estruturas tectônicas na crosta e no manto da Terra.
-
-
-
Inversăo Tomográfica com ondas Rayleigh no sul do Cráton do Săo Francisco e Faixa de Dobramentos Brasilía-Uruaçu
Authors A. Marchioreto and M. AssumpçăoNeste trabalho, determinaram as velocidades de grupo de ondas de superficie Rayleigh Rg de sismos regionais. Uma tomografia 2-D foi efetuada para quantificar as heterogeneidades laterais apresentadas pelas curvas. Por fim, a estrutura sísmica rasa (velocidade da onda S) foi obtida invertendo a velocidade de grupo regionalizada. A Figura 1 mostra os epicentros dos sismos, as estaçőes e o contorno das principais feiçőes geológicas que săo: o sul do cráton do Săo Francisco (SF) juntamente com o sul do Grupo Bambuí (BB), o suI da Faixa de Dobramentos Brasília-Uruaçu (BU), a Nappe de Empurrăo Socorro-Guaxupé (GX) e a Bacia do Paraná (PB).
-
-
-
Evidencias de Anomalias Geoquimicas na Composicao do Manto sob a Porçăo Central do Cinturăo Móvel Costeiro e Regiőes Adjacentes (SE-Brasil)
Authors I. Ludka, C.M. Wiedemann, B. Lammerer and F. SöllnerO estudo de diversos magmatitos básicos, de idade e contextos geológicos diferentes, evidenciou zaracterísticas geoquímicas marcantes, peculiares ao manto sob a porçăo central do Cinturăo Móvel Costeiro. A geoquímica das rochas básicas mostra enriquecimentos anormais nos elementos incompatíveis Ba, Sr e ETRL (elementos terras-raras leves), além de uma alta razăo inicial de Sr87/Sr86, comuns a todas as ocorręncias estudadas. O detalhamento da petrografia e do quimismo mineral foi ponto de partida para caracterizar as rochas básicas como ígneas bem preservadas, representantes de fusőes mantélicas.
-
-
-
Contribuiçăo Estratigráfica para uma Nova Visăo Paleomagnética das Formaçőes Cabeça, Longá e Poti (Bacia do Parnaíba), nas Regiőes Centro-Oeste e Nordeste do Estado do Piauí
Authors B. Vieira de Alencar, M. da Silva Borges and A.M. GóesOs dados paleomagnéticos provenientes das formaçőes Poti e Longá, na regiăo entre Floriano e Nazaré do Piauí, e das formaçőes Cabeças e Longá da regiăo de Batalha/Barras, foram reavaliados sob o ponto de vista estratigráfico. A partir de dados paleomagnéticos foram definidos 4 conjuntos estatísticos: 1) topo da Formaçăo Cabeças, 2) Formaçăo Longá, 3) base da Formaçăo Poti e 4) topo da Formaçăo Poti. Segundo as reavaliaçőes feitas nestes dados, variaçőes geométricas importantes foram constatadas, sugerindo que: 1) uma possível influęncia glacio-tectônica na Formaçăo Cabeças pode ter provocado rotaçőes/translaçőes nas camadas sedimentares; 2) a concentraçăo de direçőes medias próxima da vertical suscita a influęncia de clima mais frio durante a deposiçăo da Formaçăo Longá, possivelmente com tendęncia peripolar; 3) as variaçőes das inclinaçőes de intermediárias a baixas da Formaçăo Poti levam a idéia de uma aproximaçăo do equador durante a sua deposiçăo, talvez em clima temperado.
-
-
-
Paleomagnetism of Carbonatic Sequences from the Bambui Group-Sao Francisco Craton
Authors M.S. D‘Agrella-Filho, M. Ernesto, R.I.F. Trindade, R. Siqueira, J. Nobre-Lopes and K. KawashitaThe Neoproterozoic period corresponds to a very important phase in the evolution of the Earth, when the Rodinia supercontinent desagregated to form the Gondwana (e.g. Dalziel, 1997). Although speculative models based on geological grounds can be proposed for the evolution of these supercontinents, paleomagnetism can establish the paleogeography of the envolved cratonic units more precisely. The Sao Francisco Craton is covered by extensive carbonatic layers which were deposited during Neoproterozoic. With the purpose of establishing the paleogeography of the Congo/Sao Francisco Craton in the Rodinia and Gondwana, a paleomagnetic study of these sedimentary covers is being carried out. Results from the carbonatic sequences of the Salitre Formation (Una Group, Bahia State) was already published by D'Agrella-Filho (1995). In this paper we present preliminary paleomagnetic results from the Bambui Group (Minas Gerais State), which is possibly correlated with the Una Group.
-